This post is a quick overview of how to create a black-and-white using the GIMP.
The GIMP has several ways to convert a digital photo into black and white. The Colors menu has a Desaturate sub-menu which holds them. In this post I’ll run through them, as they are presented in the GIMP version 2.10.22.
In my post Black and White Digital photo processing-Part 1, I discuss the theory of converting color to black and white. That article might be useful to read as background.
Color-to-gray
The first tool in the group is “Color to gray”, it creates a grayscale of your image based on a complex algorithm that “will attempt to scale the values of pixels within its neighborhood (radius) to maximize contrast”. The effect tends to add some grain. While the resulting image contains a lot of detail, it appears brighter than the original color photo.
You can also adjust the parameters of radius, samples and iterations using sliders to get different effects, as in the example below where I increased all three:
Another option is to check the Enhance Shadows box:
Graininess
Color-to-gray tends to add grain to a photo. Especially if you check the Enhance Shadows box. One way to reduce the graininess you can increase the number of iterations. Since this adds to processing time, you might want to get everything else the way you like it before you increase the iterations.
Desaturate
Selecting this option brings up a pop-up menu. In this menu you can choose a method for desaturation. Each method uses a different algorithm, based on different color models. Five different methods are available.
The choice is yours! Aside from choosing the method, Desaturate is not a flexible tool, each method has a set algorithm, for flexibility you use the mono-mixer.
Mono-mixer
Mono-mixer is the GIMP’s most flexible black and white conversion tool. It uses desaturated Red, Green and Blue channels. You specify the percentage for each desaturated channel to get a custom look.
Quick note about the mono-mixer: You might find directions floating around in the inter-world about using the channel mixer with a monochrome option. These only apply to older GIMP versions. The mono-mixer replaces this method. The channel mixer in the GIMP version 2.10 does not have a monochrome option.
Channels
As each channel tends to have different characteristics, a custom blend can bring out what you want to emphasize. I highly recommend reading this article by Moro-Foto. It both describes in depth how to use this tool and includes a good description of the characteristics of the different channels.
A couple of mixes:
Preserve luminosity option
One feature you should note is the “preserve luminosity” check box. If left unchecked, it allows your chosen percentages to add to a number greater than or less than 100%. A total greater than 1.0 (100%) will result in an image lighter than the original and one less than 100% will be darker. However, If checked, the preserve luminosity algorithm will calculate a ratio to keep the total at 100%, then use the modified values to create the black and white.
Finishing up your creation
Regardless of which method you use to turn your photo to black and white, the GIMP has many filters that are not specific to black and white that can be used to add to black and white creations. For example, you can use tools in the Colors menu, like Exposure, Curves, Levels, and Brightness-contrast to give an image a bit of pop. They can be used either before or after converting to black and white.
Other filters that can be used are in the Filters menu. Some of the filters under “noise” can be used to add a look of grain. You can also use filters in the Light and shadows menu for nice effects. The example below uses the CIE Ich Noise filter to add a little bit of grain and a vignette to the Ilford film emulation:
References
While based on an older version of the GIMP, which has different options and tools, this article by Pat David is excellent, and fairly technical: GIMP-Digital B&W Conversion. This article explains some theory about how conversion works. Notable changes since this was written: the C2G GEGL procedure is now in the desaturate menu as color-to-gray. And the mono-mixer, also in the desaturate menu, replaces the channel mixer with a monochrome option. Also, there are more desaturate methods now.
Mentioned in the Mono-mixer section: Perfect black and white with Mono-mixer by Moro-Foto. This is the best description I have seen of the characteristics of the different channels and how to use the mono-mixer to take advantage of them.